Bone Heat Weighting Failed, after turning on symmetry
This kind of topology (found in your model), in which a vertex is "inside" the mesh,  is not accepted by the automatic weighting algorithm: it only works well with clean manifold meshes, without holes.
Your mesh is made of triangles, which will not deform very well, as quads can.
I suggest you to retopologise your mesh with clean quads only, then you will have no weighting problems.
If you don't want to retopo, consider manual weighting, using the function "with empty groups" during parenting, which creates an empty vertex group associated to all the armature bones marked as "deforming":
You can select all the vertices you want to assign to a bone, select the corresponding vertex group in the properties tab, choose a weight value and click assign.